1. Field of the Invention
The present invention relates to an active alignment method for a multi-channel optical transmitter and receiver, and more particularly, to an active alignment method for a multi-channel optical transmitter and receiver to reduce the sizes of a Transmitter Optical Sub-Assembly (TOSA) and a Receiver Optical Sub-Assembly (ROSA) for use in a Coarse Wavelength Division Multiplexing (CWDM) or Local Area Network-Wavelength Division Multiplexing (LAN-WDM) system and increase reliability, economic effectiveness, and reproducibility.
The present invention was derived from a study conducted as one of technology development projects for Information Technology (IT) growth engines by the Ministry of Knowledge Economy and the Institute for Information Technology Advancement (the national project's name: 100 Gbps Ethernet and Optical Transmission Technology Development).
2. Description of the Related Art
Along with an increase in the amount of data transmitted over a network, a WDM optical transmission system has been substituting for a single-channel optical transmission system. WDM is a data transmission and reception scheme in which data at different wavelengths are multiplexed and transmitted/received through a single optical fiber.
For 40 G and 100 G Ethernet, standardization of a Physical Medium Dependent (PMD) sublayer using CWDM and LAN-WDM is under way. Compared to Dense WDM (DWDM), CWDM is characterized by a reduced number of data channels transmitted on a single optical fiber.
A metro access network near to a subscriber network may not require as much traffic as a core network. Over the metro access network, a CWDM optical network system is widely used instead of a DWDM optical network system that suffers from a narrow gap between data channels, complex implementation, and high cost.
Due to a smaller number of multiplexable wavelengths and a larger gap between data channels than in DWDM, CWDM boasts of low implementation complexity and low design and fabrication costs.
Accordingly, low-price uncooled light sources are usually used in a CWDM optical network system, and Electro-absorption Modulation Lasers (EMLs) in a LAN-WDM optical network system.
The Institute of Electrical and Electronics Engineers (IEEE) 802.3ba 40 GBASE-LR4 Ethernet standard defines four wavelengths (1271 nm, 1291 nm, 1311 nm and 1331 nm), a data rate of 10.3125 Gb/s for each channel, and a 20-nm channel gap, for CWDM.
According to the IEEE 802.3ba 100 GBASE-LR4 Ethernet standard, four wavelengths (1295 nm, 1300 nm, 1305 nm and 1310 nm), a data rate of 25.78125 Gb/s for each channel, and a 5-nm channel gap are defined for LAN-WDM.
CWDM and LAN-WDM each require a TOSA for multiplexing data channels and transmitting them on a single optical fiber and a ROSA for demultiplexing a multiplexed optical signal into individual optical signals. The TOSA is provided with light sources having four different wavelengths. To multiplex data channels emitted from these light sources onto a single optical fiber, the TOSA requires a fiber optic coupler, an optical Multiplexer (MUX), and an optical signal generator.
The optical signal generator converts electrical signals of data channels to optical signals. The optical MUX multiplexes the optical signals received from the optical signal generator to a single optical signal. The fiber optic coupler, which is physically connected to an optical fiber, transmits the multiplexed optical signal.
The fiber optic coupler, the optical MUX, and the optical signal generator are in passive or active alignment to one another in order to minimize the loss of light generated from the optical signal generator. The passive alignment scheme is to align the fiber optic coupler, the optical MUX, and the optical signal generator fixedly at predetermined positions of a substrate. The active alignment scheme is to align the fiber optic coupler, the optical multiplexer, and the optical signal generator according to a predetermined beam pattern by use of an additional alignment device or a laser welding device, or manually, taking into account light intensity, distances among the fiber optic coupler, the optical multiplexer, and the optical signal generator, and positions that maximize the optical power of an optical signal.
Despite the advantages of simple alignment and packaging of the fiber optic coupler, the optical MUX, and the optical signal generator, the passive alignment scheme has low accuracy and low reliability. On the other hand, the active alignment scheme increases time and cost because optical power and a beam pattern are to be adjusted for the respective components.
Many methods have been considered to implement a TOSA and a ROSA for CWDM and LAN-WDM systems. A method for performing an optical multiplexing function using beam splitters is disclosed in U.S. Pat. No. 7,184,621 (hereinafter, referred to as reference 1). In reference 1, four light sources, three beam splitters, an isolator, and a collimator are provided and the light sources are distributed according to the positions of the beam splitters.
However, a TOSA is difficult to miniaturize and packaging cost increases due to optical alignment in reference 1.
U.S. Pat. No. 7,218,806 (hereinafter, referred to as reference 2) discloses an optical transceiver using an optical waveguide and a thin-film filter on a platform configured with a Planar Lightwave Circuit (PLC). The optical transceiver of reference 2 may contribute to miniaturization of a TOSA and a ROSA. However, the optical transceiver also increases the cost of optical alignment packaging of a light source, an optical detector, and an optical waveguide and decreases product reproducibility. The optical transceiver is not favorable in terms of mass production.